1. Field of the Invention
The present invention relates to improvement of polyamide resins. More particularly, the invention relates to a highly impact resistant polyamide composition containing an acid modification product of a polyolefin elastomer as a modifier and to a less hygroscopic and highly impact resistant polyamide composition containing a novolak phenol as well as an acid modification product of a polyolefin elastomer as modifiers.
2. Description of the Prior Art
Polyamide resins (PAs) are polymers having amide functionality in the monomeric unit thereof, and generally referred to as "nylons". Examples thereof include polyhexamethylene adipamide (nylon 66), polycaprolactam (nylon 6, PA 6), polyundecanamide (nylon 11) and polylaurylactam (nylon 12).
The polyamides are less expensive, and superior in moldability, mechanical properties, chemical properties (e.g., erosion resistance and corrosion resistance) and physical properties. The polyamides are widely used as materials for various mechanical components of automobiles and electric appliances.
Research and development have been made on compounding materials and modifiers which are used for improvement of properties of the polyamides to meet property requirements of final products with little or no impairment of the excellent properties intrinsic in the polyamides.
The polyamides have a drawback such that they generally have a low impact resistance in an absolutely dry state (e.g., immediately after the molding thereof), unlike in a moistened state.
A current technical challenge is to improve the impact resistance of the polyamides without sacrificing any of their excellent intrinsic properties. If the above mentioned material is developed, such an inconvenience that a product molded from a polyamide composition is fractured due to a low impact resistance thereof in a dry state can be eliminated. Further, a composition such as of a less expensive nylon 6 can be imparted with a high impact resistance so as to be used as a material for highly impact resistant products. Such a polyamide composition will be a promising alternative to more expensive impact resistant materials.
However, relatively heavy limitations are imposed on the choice of the modifier to be contained in the polyamide composition, because the excellent properties of the polyamide are attributable to amide functional group (functionality) thereof. Such limitations are associated with the melting point and glass transition point of the modifier, and the injection-moldability and other process requirements of the resulting composition, which depend on products to be molded from the composition.
Therefore, it is difficult to find the compounding material or the modifier for improving the impact resistance of the polyamide compositions without sacrificing the excellent properties of the polyamides.
For improvement of the impact resistance of a polyamide in the absolute dry state, polyolefin polymer, such as ethylene/propylene copolymer or ethylene/propylene/1,4-hexadiene copolymer (EPDM), grafted by maleic anhydride or the like are used as an impact modifier. In such a case, the resulting polyamide exhibits an improved impact resistance, but the flexural modulus thereof is remarkably reduced. Therefore, such a polyamide is not suitable for applications requiring a sufficient load resistance and impact resistance.
Among the exemplary polyamides, the nylon 6 (polycaprolactam, PA 6) is less expensive and excellent in chemical resistance and mechanical properties. However, the nylon 6 absorbs a large amount of water because of the presence of amide functional group in the molecule thereof, so that the nylon 6 is liable to be softened. Accordingly, the tensile strength, flexural strength and flexural modulus of the nylon 6 are remarkably reduced in a moistened state.
More specifically, it has been found that the tensile strength, flexural strength and flexural modulus of the nylon 6 in a moisture conditioned state (under equilibrium conditions at 23.degree. C., 50% RH) are about 60% to 40% lower than in the absolutely dry state, though the Izod impact strength is two times higher. This is demonstrated in Comparative Example 2-1 of the present invention shown in Table 2-1. The reduction in the flexural modulus means that deformation of a molded product under load is aggravated due to moisture absorption thereof. Therefore, it is difficult to employ the nylon 6 as a material for structural components. For this reason, there is a demand in the industries for improvement of such defect of nylon 6.
A glass fiber reinforced polyamide composition of improved properties is disclosed in Japanese Unexamined Patent Publication No. 7-53862 (1995) which was filed by the inventors of the present invention. The polyamide composition comprises glass fibers and a polyamide matrix (PA/Novolac phenol composition, PA/PH composition) containing nylon 6 and a novolak prepolymer. In the case of the PA/PH composition, the reduction of the tensile strength and flexural strength due to water absorption is suppressed, but the Izod impact strength is reduced in comparison with the case of the nylon 6. Where the PA/PH composition contains the novolak prepolymer in a proportion of 30%, the Izod impact strength thereof in the absolute dry state is much lower than that of the nylon 6 (less than 50% of that of the nylon 6). This is demonstrated by Comparative Examples 2-1 to 2-4 of the present invention shown in Table 2-1. For this reason, the glass fibers are an indispensable component of the polyamide material for the composition.